Performance of an electrodeionization apparatus depends on the rejection/blockage/encapsulation of impurity ions in the ion-concentrating compartments, that have migrated from the ion-depleting compartments through their respective ion permeable membranes. The ion-concentrating compartments in an electrodeionization apparatus may be filled with either: (a) water, or an aqueous solution, which receives the impurity ions, or (b) water, or an aqueous solution, in conjunction with some ion-conducting materials. When the ion-concentrating compartment spacers are filled with non-ionconductive mesh, the ions from the ion-depleting compartments directly enter the liquid phase of the ion-concentrating compartments at the membrane surface. Solution mixing, ion migration, and ion diffusion take place to provide a homogeneous liquid at a distance from the membrane surfaces. When the ion-concentrating compartments contain ion-conductive materials, the impurities that have migrated out of the ion-depleting compartments will remain in their respective ion-conductive phases until the impurities either: (a) perform ion-exchange with another ion from solution, or (b) are transferred to the liquid phase, by migration, along with a corresponding oppositely charged ion from the oppositely charged ion-exchange material.
For the normal salt ions, such as: Na+, K+, Li+, etc. and Cl−, Br−, NO3−, SO42−, etc., migrating into the ion-concentrating compartments, their concentration merely increases in the liquid flowing through the ion-concentrating compartments.
For acidic and basic ions, the transition of these ions into solution from the ion exchange material phase can cause an acid/base reaction to occur, forming a neutral compound. For example, the following reactions may occur:H+(aq)+OH−(aq)→H2O(l)H+(aq)+CH3COO−(aq)→CH3COOH(aq)  (acetic acid formation)H+(aq)+HCO3−(aq)→H2CO3(aq)→CO2(aq)+H2O(l)  (carbonic acid formation)NH4+(aq)+OH−(aq)→NH4OH(aq)→NH3(aq)+H2O(l)  (ammonia formation)
In cases where the positive and negative ions entering the solution phase form a compound with low solubility, high local concentration levels can be developed, leading to the formation of a precipitate. For example, the following mechanisms may be operative:Ca2+(aq)+CO32−(aq)→CaCO3(s)andMg2+(aq)+2OH−(aq)→Mg(OH)2(s)
Controlling the location where these scaling reactions occur is vital to both the ability of an electrodeionization apparatus to produce high resistivity product water and to reduce scale formation in the ion-concentrating compartments. The effect of the configuration of the ion exchange material in the ion-concentrating compartment on the product water resistivity arises from the fact that neutral species formed in the ion-concentrating compartments from acid/base neutralization reactions are not rejected by one of the two ion-permeable membranes. The formed neutral species, such as CO2, CH3COOH and other weak acids, can diffuse through a low pH cation permeable membrane, but if they come into contact with a high pH anion permeable membrane, they are ionized and rejected. Similarly, weak bases such as NH3 are not rejected by the anion permeable membranes and are able to diffuse through the anion permeable membranes, but become ionized when in contact with a low pH cation permeable membrane. The rate at which this back diffusion (e.g. CO2 or CH3COOH through a cation permeable membrane, or NH3 through an anion permeable membrane) occurs is dependent on the local concentration of the species in the ion-concentrating compartment and its location with respect to the membrane surface, and also the intrinsic membrane properties.
In an electrodeionization apparatus containing inert mesh filled ion-concentrating compartment spacers, the concentrations of these neutral weak acid and weak base species do not build up at the membrane surfaces and their back diffusion into the ion-depleting compartments is minimal. However, this is not the case in an electrodeionization apparatus with ion-exchange material filled ion-concentrating compartments. With ion exchange material structures of pure anion, pure cation or mixed bed ion exchange resins, weak acid and base impurity species can travel within an ion exchange material phase all the way through the thickness of the ion-concentrating compartment to the surface of the opposing ion permeable membrane. At this interface, the ion can encounter either a hydronuim ion (cation permeable membrane surface) or a hydroxide ion (anion permeable membrane surface) and form a neutral molecule. Due to the low linear velocity at the membrane surface, a high concentration of these neutral species can form, creating a large driving force for the back diffusion of these species into the adjacent ion-depleting compartments. Once these species are transported into the ion-depleting compartments through this back diffusion, ionization occurs which reduces the product water resistivity.